This invention relates generally to a lean NOx catalyst system for treatment of diesel engine exhaust to reduce nitrogen oxide (NOx) emissions and more specifically to a metal doped oxide catalyst having high deNOx selectivity for NOx reduction and a method for preparing the metal doped oxide catalyst. The preferred oxide support material is xcex3-alumina.
Due to primarily federal regulations, engine manufacturers are being forced to reduce the amount of harmful compounds in the combustion exhaust, including a reduction in nitrogen oxides or NOx. To effectively reduce the NOx concentrations in the exhaust stream of lean burning engines including diesel and certain spark ignited engines, the use of selective catalytic reduction of NOx with a hydrocarbon in oxygen rich conditions (lean NOx catalyst systems) is actively being pursued.
Unfortunately, many catalysts or catalytic systems that have been utilized for lean burn engines to date suffer from low removal of NOx, inadequate catalyst durability, low thermal stability and a limited temperature window of operation. For example most commercially available lean NOx catalytic systems for lean burn engines only achieve less than 20% NOx reduction as a passive system and possibly up to 40% for active systems (i.e. when supplemental hydrocarbon reductant is introduced into the exhaust stream). In addition, most commercially available lean NOx catalytic systems also are subjected to sulfur poisoning, from the minimal amounts of sulfur found in many fuels and lubricants, resulting in low catalyst durability.
Such active catalytic systems involving lean NOx catalysts require a sufficient concentration of hydrocarbon (HC) species to be present. In other words, in combustion exhaust purification systems having an oxygen environment above 4% concentration, some type of reducing agent, usually a hydrocarbon compound such as diesel fuel, must be introduced into the exhaust in order to achieve acceptable reduction levels of NOx compounds. However, such introduction of fuel tends to reduce the overall fuel efficiency of the engine system.
Among metal oxide materials, alumina has been known as a durable material as well as a promising catalyst for lean-NOx reactions at high temperature. Various prior art disclosures have reported that alumina showed high deNOx performance vis-à-vis NO2, particularly when oxygenated hydrocarbons were present in the exhaust gas stream. However, exhaust gas streams from most diesel engines also include NO constituent that must be readily converted.
What is needed therefore, is a lean NOx catalyst that has good selectivity of both NO and NO2 in order to achieve both an improvement in the reduction of NOx from the engine exhaust stream with minimal fuel efficiency impact on the overall engine system. The presently disclosed catalyst is directed to overcome the problems as set forth above.
In one aspect, the disclosed invention is a lean NOx catalyst comprising a ceramic substrate an oxide support material deposited on the substrate; a metal dopant introduced into the oxide support material such that the lean NOx catalyst includes impurities and/or thermal stabilizers in the amount of 0.2 weight percent or less. The metal promoters or dopants are selected from the group consisting of indium, gallium, tin, silver, germanium, gold, nickel, cobalt, copper, iron, manganese, molybdenum, chromium, cerium, and vanadium, oxides and combinations thereof.
In another aspect, the disclosed invention is a lean NOx catalyst comprising a ceramic substrate; a xcex3-alumina support material deposited on the substrate; and a metal based promoter introduced into the xcex3-alumina wherein the xcex3-alumina support material has a surface area greater than about between about 80-350 m2/g and preferably greater than 200 m2/g; and a pore size of between about 3-30 nm and preferably greater than 12 nanometers. The metal promoters are preferably selected from the group of indium, gallium, tin, silver, germanium, gold, nickel, cobalt, copper, iron, manganese, molybdenum, chromium, cerium, vanadium, oxides and combinations thereof.